1. Field of the Invention
This invention relates to a diffuser for a centrifugal compressor, and, more particularly, to a diffuser suitable for use in a high speed centrifugal compressor in which high pressure ratio can be obtained by a single stage.
2. Description of Prior Art
Generally, a centrifugal compressor generates a high speed air flow by the rotation of an impeller thereof. In a case of a high speed centrifugal compressor which is so designed that high pressure ratio can be obtained through a single stage, since the speed of air discharged from the impeller exceeds sonic velocity, a diffuser having stator blades is provided in an area outside the impeller, with this outside area corresponding to the downstream of the impeller, for the purpose of converting kinetic energy of the fluid discharged from the impeller into pressure energy.
A plurality of stator blades forming the diffuser is provided in a peripheral portion or outside area of the impeller, and spaces between these stator blades form diffuser passages.
In the above-described type of compressor, when the rotational speed is relatively high and discharge (flow rate) is also relatively small, a separation flow region is generated in the negative pressure side of the stator blade, as a result of which, a problem arises such that the surge phenomenon is generated in which sufficient rise in pressure cannot be obtained. A diffuser for high speed centrifugal compressor which can overcome the above-described problem and in which the surging phenomenon is prevented even if in the high speed and small discharge state, was proposed and disclosed in Japanese Patent Laid-Open No. 57-159998, in which fluid passing through a diffuser is controlled by a rotatable sub-blade at the inlet portion of the diffuser for the purpose of controlling the fluid passing through the diffuser. Furthermore, in Japanese Patent Laid-Open No. 53-119411, it is proposed that a blade-provided diffuser is formed in a double circular blade cascade and the length of the blade in the inner annular blade cascade is arranged to be no more than 0.9 times of the interval of the blades.
However, in the former case, since the diffuser passage formed between stator blades is drastically enlarged immediately behind the downstream of the sub-blade, a problem arises such that pressure loss is generated, and the choking flow is reduced, causing the performance of the diffuser to be deteriorated. Meanwhile, in the latter case, although the above-described problem of reduction in choking flow does not arise, a loss is generated due to a strong shear flow generated at the downstream of the blades of the inner circular blade cascade when the speed of the fluid at the inlet portion of the blades which form the inner circular blade cascade exceeds sound velocity. As a result, a problem arises such that the performance of the diffuser is deteriorated.